This invention relates, in general, to perforating a subterranean wellbore using shaped charges and, in particular to, a detonation transfer subassembly that is installed within a work string between loaded perforating guns to provide an area through which the work string may be severed without the potential for detonating the shaped charges carried in the perforating guns.
Without limiting the scope of the present invention, its background will be described with reference to perforating a subterranean formation using shaped charge perforating guns, as an example.
After drilling the section of a subterranean wellbore that traverses a formation, individual lengths of relatively large diameter metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. This casing string increases the integrity of the wellbore and provides a path for producing fluids from the producing intervals to the surface. Conventionally, the casing string is cemented within the wellbore. To produce fluids into the casing string, hydraulic opening or perforation must be made through the casing string, the cement and a short distance into the formation.
Typically, these perforations are created by detonating a series of shaped charges located within the casing string that are positioned adjacent to the formation. Specifically, numerous charge carriers are loaded with shaped charges that are connected with a detonating device, such as detonating cord. The charge carriers are then connected within a tool string that is lowered into the cased wellbore at the end of a tubing string, wireline, slick line, coil tubing or the like. Once the charge carriers are properly positioned in the wellbore such that shaped charges are adjacent to the formation to be perforated, the shaped charges are detonated. Upon detonation, each shaped charge creates a jet that blasts through a scallop or recess in the carrier, creates a hydraulic opening through the casing and cement and then penetrates the formation forming a perforation therein.
It has been found, however, that it may sometimes be necessary to shut in a well due to an out of control well situation while the tool string, including the perforating guns, is disposed within the well. For example, during a snubbing operation or after the well has been perforated. If live shaped charges remain in the perforating guns, it is possible that closing a set of shear rams on a live shaped charge or other explosive components could result in a detonation. If such a detonation occurs, the live shaped charge may fire causing damage and injury to well equipment and personnel.
A need has therefore arisen for an apparatus that can be installed within the tool string between the loaded perforating guns to provide an area through which the tool string may be severed without the potential for detonating the shaped charges carried in the perforating guns. A need has also arisen for such an apparatus that can transfer detonation from one perforating gun to the next perforating gun such that the perforating guns may be fired in sequence.
The present invention disclosed herein comprises a detonation transfer subassembly that can be installed within a tool string between two detonation activated tools, such as live perforating guns, that provide an area through which the tool string may be severed without the potential for detonating the detonation activated tools. The detonation transfer subassembly of the present invention also provides for the transfer of detonation from one detonation activated tool to another detonation activated tool such that the detonation activated tools may be detonated in sequence.
The detonation transfer subassembly for the present invention comprises a first explosive carrying member and a second explosive carrying member. Each of these explosive carrying members has an explosive disposed therein. For example, the first explosive carrying member may have an explosive train including one or more boosters, a detonation cord and an unlined shaped charge. Similarly, the second explosive carrying member may have an explosive train including an initiator, one or more boosters and a detonation cord.
Disposed between the first and second explosive carrying members is a detonation transfer member. The detonation transfer member has a longitudinal passageway. In one embodiment, the detonation transfer member may include a barrel disposed within a housing such that a vent chamber is defined therebetween. In this embodiment, the longitudinal passageway is disposed within the barrel. In addition, the barrel may include one or more vent ports that create a communication path between the longitudinal passageway and the vent chamber.
A firing pin is disposed within the longitudinal passageway. The firing pin has a first position proximate the first explosive carrying member and a second position proximate the second explosive carrying member. The firing pin may be propelled from the first position to the second position in response to, for example, gas pressure generated by detonating the explosive disposed within the first explosive carrying member. Alternatively, a solid rocket propellant or other suitable propellant may be used or wellbore fluid pressure may be routed to the fire pin. In such an event, the firing pin impacts the explosive disposed within the second explosive carrying member, thereby transferring detonation from the first explosive carrying member to the second explosive carrying member.
To assure that the firing pin impacts the explosive disposed within the second explosive carrying member with sufficient force to detonate this explosive, the first explosive carrying member may include an expansion chamber for the gas generated from the detonation of the explosive or ignition of a propellant in the first explosive carrying member. In addition, the firing pin may be initially fixed relative to the barrel by a shear pin that selective prevents movement of the firing pin relative to the barrel until the force is sufficient to shear the shear pin. Finally, as the firing pin travels from the first position to the second position, air in the longitudinal chamber vents to the vent chamber to avoid creating unnecessary resistance to the movement of the firing pin.
As such, the detonation transfer subassembly of the present invention provides a region through which a tool string may be severed between two detonation activated tools that without the potential for detonating the detonation activated tools. Also, the detonation transfer subassembly of the present invention provides for the transfer of detonation from one detonation activated tool to another detonation activated tool through the detonation transfer member.
The method of the present invention for operating the detonation transfer subassembly involves, disposing a detonation transfer member between first and second explosive carrying members, creating a detonation within the first explosive member, propelling a firing pin from a first position proximate the first explosive carrying member to a second position proximate the second explosive carrying member through a longitudinal passageway in the detonation transfer member and impacting an explosive disposed within the second explosive member with the firing pin, thereby transferring detonation from the first explosive carrying member to the second explosive carrying member.
The method of the present invention for severing a work string between two detonation activated tools involves disposing a detonation transfer subassembly between the two detonation activated tools, positioning the detonation transfer member of the detonation transfer subassembly adjacent to shear rams of a blowout preventer and closing the shear rams of the blowout preventer, thereby severing the work string between the two detonation activated tools.